Fiber optic cables have become widely used in the communications industry as a means to communicate massive amounts of information. A fiber optic cable has a very high bandwidth, as compared with conventional transmission lines such as copper wire. A relatively small cable is capable of transmitting large amounts of information. Fiber optic cables are thus very desirable for use in densely populated metropolitan areas. Fiber optic cable is typically installed in metropolitan areas by burying a three to four mile "loop" or "fiber ring" that extends from a "central office" around a highly populated area. A high fiber count cable, referred to as a "backbone," is used for the fiber ring. Along the ring, smaller fiber optic cables, referred to as "customer drops" or "spurs," are spliced to fibers in the backbone and run to "termination sites" or buildings of customers.
Splicing of a fiber optic customer drop cable to a fiber optic backbone cable is made in an underground vault or manhole. The task of splicing requires much labor and time. Each optical fiber in the customer drop must be individually fused or mechanically connected to an optical fiber in the backbone cable. The splice then must be sealed within a water-tight enclosure commonly referred to as a splice closure.
One widely used type of splice closure utilizes a tubular, plastic housing sealed at each end by caps. The interior tubular housing between the caps forms a splice chamber. The closure must have an air-tight seal to prevent entry of water, dust and other contaminants which may potentially damage a fiber optic splice. The end caps typically take the form of a "duct closure" or seal. A duct seal is formed of a resilient, compressible, disc-shaped material, such as rubber or polyurethane. Two plates located on opposite sides of the material compress the material, causing it to expand radially and, consequently, to snugly engage the inner walls of the tubular housing. The seals include holes and slots through which the fiber optic backbone extends to enter the enclosure. These holes and slots are sealed by the expansion of the resilient material during compression. The duct seal has the advantage of allowing the closure to be reentered and sealed quickly and repeatedly for splicing, servicing and maintenance. To help organize individual optical fibers for splicing, splice "trays" are provided for separating out and holding each individual fiber. These trays are stacked within the closure and held by means of an elongated bracket which extends the length of the tubular housing. The seals are mounted to opposite ends of the bracket. Also mounted to the bracket is a "pull" bracket to which a metallic strength member in the cable is secured to assist in preventing damage to the splice in the event the cable is pulled. Thus, an integrated unit is formed with the end cap seals, brackets and trays. Accessing the closure requires only loosening the compression plates on the seals and sliding the tubular housing off.
It is sometimes necessary to locate a fiber optic cable for repair, replacement or maintenance, to sort it from many other cables which may be present underground, or to mark the ground above the line during periods of construction in the vicinity in order to avoid inadvertently damaging the cable. Most fiber optic cables have either a conductive metallic sheath or tracer wire embedded in the cable which enables a low frequency, electrical signal, called a "tone," to be transmitted from the central office. The sheath or tracer wire cable acts, in essence, like a very large antenna. A direction-finding receiver is used to locate the cable. In order to conduct the tone signal along the cable, the sheath of the fiber optic cable cannot be grounded at any point except at a terminating end. Otherwise, the signal will not travel the entire length of the cable.
Occasionally, the fiber optic cable must be strung over major roadways and other obstacles. The conductive metallic sheath or tracer wire is thus subject to transient electrical surges induced by lightning, as well as to other types of induced signals such as those caused by electromagnetic fields surrounding high voltage power lines. Electrical surges on the cable can cause damage to equipment connected to the cable, both at the customer end and at the central office. A traditional method for suppressing surges on the cable is to ground the cable by bonding the sheath directly to a ground wire that is attached to a ground rod in each manhole. Transient electrical surges on the sheath thus never reach the customer. Unfortunately, directly grounding the sheath also grounds the cable-locating signal. Locating a cable thus requires the ground to be physically "lifted" or disconnected from the sheath or tracer wire in each of the manholes. Manually lifting the grounds is a laborious, time-consuming task in highly populated areas where there may be splices every few hundred meters.
Another method is to "float" the backbone cable and customer drops. The sheath of the customer drops is bonded to the sheath of the backbone cable. A surge suppressor is installed at the termination site of each customer drop by connecting it between the outer sheath of the cable and a ground bus bar in the building. The surge suppressor conducts when the voltage on the cable exceeds a predetermined voltage, pulling the sheath to ground. Otherwise, it behaves like a capacitor which passes high frequency signals to ground but blocks direct current and low frequency signals, including cable-locating signals. A resistor is also placed in parallel with the surge suppressor, between the cable and ground, to create an impedance with respect to periodic signals such as the cable-locating signal. A small amount of the power of the cable-locating signal travels up the customer drop, with the remaining power propagating down the backbone.
This second method has two drawbacks. First, it permits transient electrical surges to travel to the user's premise before being grounded. Generally, it is preferable to keep transient electrical surges as far away from the protected equipment as possible. Some municipal building codes also require that all fiber optic cables be grounded in the manhole. Second, it requires installation of a surge suppressor at each termination site, thus increasing installation time and cost.